Tetrahydrofuran (THF) is a useful chemical that can be used as a high-purity solvent, or be polymerized to form polytetramethylene oxide. This polymer can be used to make elastomeric polyurethane fibers like Spandex. Maleic and/or succinic diesters can be hydrogenated over copper-based catalysts in the gas phase at elevated pressures to give mixtures of THF, gamma-butyrolactone (GBL), and 1,4-butanediol (BDO). Byproducts of this hydrogenation/hydrogenolysis process include n-butanol and its derivatives, including dimethyl ether, dibutyl ether, butyraldehyde, and linear ethers of butanol with the esterifying alcohol.
The main products of this process are desired intermediates for important and useful chemicals. GBL is used for preparing pyrrolidones such as pyrrolidone itself and N-methylpyrrolidone. BDO finds use in the production of polyurethanes and polyesters, and, importantly, is used for preparing THF. Typically, the complete process for the synthesis of THF is as follows: First, maleic anyhdride undergoes esterification with methanol (or ethanol) to form dimethyl maleate (DMM). Then, DMM is hydrogenated to form BDO and methanol (or ethanol). This step has been carried out using a catalyst comprising copper, manganese and alumina. Finally, the resultant stream containing BDO is converted, to whatever extent required, via dehydration and ring closure, to THF. This step is typically carried out using copper on alumina catalyst.
However, during the DMM to BDO reaction, several unwanted byproducts are formed that include butanol, and butyraldehyde. The total amount of THF formed from BDO and the total consumption of BDO to THF depends on the reaction conditions; and total amounts of THF from 10 mol % and higher may be formed. However, in such a reaction environment, due to the presence of butanol and methanol, the THF formed is accompanied by the formation of small amounts of butyl methyl ether (BME). The various uses of THF require very pure THF that is especially free from even low levels of BME or dimethyl ether (DME). BME even in very small amounts affects the use of THF, as it is very difficult to distill from the desired THF product. This difficulty in distillation can lead to great expense to obtain the necessary THF purity.
Thus, there is a need for a catalyst that can convert 1,4-butanediol to THF while minimizing the amount of byproducts (particularly BME), such that the cost in distillation of the THF can be greatly reduced.